This invention relates to optical data communication systems and, more specifically, to devices and methods for providing efficient use of optical fibers needed for full duplex data communications in a synchronous optical network (SONET).
The use of optical fibers for data transmission is increasing. Optical fibers are now used for data communications in local area networks, national and international telecommunications, cable television distribution, and in other communications services. Although the cost of optical fiber has been significantly reduced in recent years, the cost of fiber installation is still high. Accordingly, there is a need to use installed fibers in a more efficient manner.
In many fiber transmission systems, conversions are made between optical signals and electrical signals. For example when telecommunication signals, such as DS3 signals, are multiplexed for fiber transmission, the DS3 signals are multiplexed and sent to an optical transmitter. The optical transmitter converts the electrical signals to optical signals using electrical to light conversion devices such as light emitting diodes or laser diodes. The optical signal is then directed to one or more receiving stations for conversion back to electrical signals. The optical receiver that converts the received optical signal to an electrical signal must provide relatively error free DS3 signals at each receiving station.
The continued improvement in optical components allows for more efficient use of fibers. It is not unusual to find data rates over fiber greater than several hundred megabits per second. The bandwidth capacity of optical fibers makes them suitable for use in a local area network (LAN), in LAN-to-LAN connections, in telecommunications applications, and in links from optical networks to loop access systems.
A SONET ring may serve as a wide area network (WAN)-to-WAN connection link. In a standard implementation of a SONET ring, two fibers are used to provide full duplex communications. These are conventionally referred to as xe2x80x9cactivexe2x80x9d fibers. In addition, technical standards adopted for commercial SONET rings require a xe2x80x9cone-to-onexe2x80x9d fiber redundancy to protect the network against failure of one or both of the active fibers. To provide this protection, a SONET ring will also have two protection fibers for backup, for a total of four fibers. Normally, the protection fibers carry no data, only SONET framing information and idle fields. Thus, in a conventional SONET ring, the capacity of the fibers is underutilized because of the requirement for protection fibers. One example of a prior art implementation of a SONET ring optical fiber system 100 is shown in FIG. 1. Three conventional SONET access loops 106A, B, and C are coupled together via SONET access (add/drop) multiplexers 104 and a synchronous optical network (xe2x80x9cSONETxe2x80x9d) ring 102A, B, and C, using four fibers as the coupling link. SONET technologies are well known in the art, as specified in various industry standards such as Bellcore GR-253; ANSI TI305, and ITU G-870-875. The SONET ring connection of FIG. 1 conventionally uses four fibers, two active fibers for duplex data communications and two protection fibers reserved for backup and protection of the SONET ring. The SONET access loops 106A, B, and C may be a wide area network (WAN), a digital loop carrier (DLC), a cable TV distribution network, etc., and may have data rates in the hundreds of megabits per second. The SONET ring 102A-C coupling the SONET access loops 106A, B, and C may be operated at a variety of data rates depending on the design limitations and the needs of the user. SONET ring data rates vary from near 50 megabits per second to in excess of several gigabits per second.
In linking a local loop access system, private network, or other network element to a SONET ring, the owner of the loop access system, private network, or network element must make a decision as to whether to continue the redundant SONET ring fiber topology within that link. In making that decision, the availability and cost of installing and/or leasing the fibers is an important factor. As demand for data and data rates increases, a more efficient use of fibers can be of benefit to service providers and users. In some network-to-local loop access systems, such as the Total Access system available from ADTRAN, Inc. of Huntsville, Ala., two fibers are used to couple a SONET ring to an access interface module. Unfortunately, conventional devices and systems that provide communications links to SONET rings have not given a user the option of implementing that link over a single fiber while still providing full duplex communications.
It is a principal object of the present invention to provide an apparatus and means for more efficient use of optical fibers in a communications link to a SONET ring, thereby freeing fibers for other services.
Another object of the present invention is to provide for a more efficient use of optical fibers without reducing the quality of service.
Yet another object of the present invention is to increase the efficiency of fiber utilization by using standard optical and electrical components that provide a cost effective and reliable solution.
The present invention implements these and other objectives by multiplexing the four fibers in a SONET ring into a single linking fiber for full duplex, multichannel optical transmission. To accomplish this, a fiber quadrupler device and method of this invention optically couples two of the SONET fibers to two data inputs of a transmit converter, for communications of downstream data from the SONET ring to a network element linked to the quadrupler by a single linking fiber. The two remaining SONET fibers are optically coupled to two data outputs of a receive converter, for communications of upstream data from the network element over the single linking fiber, to the SONET. An optical transceiver processes the downstream transmit signals from the transmit converter and directs them across the linking fiber to the network element. The optical transceiver also receives the upstream optical signals sent from the network element over the linking fiber and directs them to the receive converter.
In a first embodiment of the fiber quadrupler device, the optical downstream and upstream data signals are converted into electrical signals. RF subcarrier modulation (SCM) is used in the transmit converter to combine the downstream data from the two data input fibers into a single multiplexed electrical signal. Conversely, RF demodulators in the receive converter separate the RF subcarriers in the multiplexed upstream signal so that the data can be directed to the corresponding data output fibers.
In a second embodiment of the fiber quadrupler device, the upstream and downstream data are processed only as optical signals, using wavelength division multiplexing (WDM) in the transmit converter to multiplex the downstream data signals and wavelength selective demultiplexing of the upstream data signals sent over the linking fiber from another network element.
Preferably, the upstream and downstream data signals sent to and received from the SONET ring are amplitude modulated using conventional digital modulation techniques (e.g., pulse amplitude modulation or amplitude shift keying) for actual communication of data over the RF/optical carriers and subcarriers.